1. Field of the Invention
The invention concerns isosorbide-modified unsaturated polyester resins and particularly formulations comprising the isosorbide-modified unsaturated polyester resins and low profile additive (“LPA”). The formulations can be incorporated into molding compounds, for example sheet molding compounds (“SMC”) and bulk molding compounds (“BMC”). The molding compounds can be applied in fiber reinforced composites (“FRC”). The formulation may also be applied in making laminates and the like. The FRC and laminates and other similar materials comprising the isosorbide-modified unsaturated polyester resins and LPA will produce low shrink matrices.
2. The Related Art
Unsaturated polyester resin (“UPR”) and vinyl ester resin (“VER”), which are solutions comprising reactive monomer such as styrene, reinforced with glass fibers are used extensively as components of FRC. Cured FRC made from these compounds offer numerous advantages over their metallic counterparts, such as lower weight, lower coefficients of thermal expansion, higher thermal damping, better design flexibility, and the like. For example, due to their low specific gravities, FRC possess superior strength-to-weight and modulus-to-weight ratios relative to metals. Furthermore, FRC enables manufacturers to consolidate multi-component metal parts into one composite part, which results in considerable manufacturing and assembly cost reductions.
Isosorbide is referred to as a “sugar diol” because it is derived from D-glucose which ultimately is generated from starch. Isosorbide can be included as a glycol into unsaturated polyesters using standard polyester synthetic techniques.
As a result of the advantages of FRC or other composite materials, these materials are desired for use in many industries. For example, FRCs are applied in the transportation industry as a light weight, cost effective replacement for metal to result in lower cost and greater fuel efficiency vehicles, such as automotives, trucks, airplanes, boats, train cars and the like. Other industries where FRC is used include the manufacture of plumbing fixtures like piping, bath and shower surrounds, countertops and the wind energy industry. Composites based on infusion technology can be applied in various aspects of the transportation industry, including in boats and other watercraft.
One of the factors that influence several important properties of thermosetting composites, such as FRC, is negative volumetric change (shrinkage) during cure. UPRs and VERs cure via addition polymerization reaction of all reactive functionalities to produce crosslinked thermosetting material. During this process, shrinkage may be caused by several factors, one of which is that before the commencement of the polymerization (e.g. in the monomer form) molecules are positioned at a van der Walls distance from one another, while after the cure (in the polymer form) the corresponding monomeric units are joined covalently and move closer to each other. Therefore, monomeric units are closer to each other in the polymer form than in the monomer form, which results in the net volume shrinkage. Other lesser but still contributing factors, are change in entropy in going from monomer to polymer and the free volume and efficiency of monomer and polymer packing.
The degree of shrinkage is dependent on the polymerization temperature. Generally, room temperature cure conditions result in less shrinkage than if the cure is performed at elevated temperatures. For example, shrinkage during SMC molding (˜300° F.) is more extensive due to effects of thermal expansion. If post-cure of a composite part is required during manufacturing or processing, then additional shrinkage may occur. This is attributed to the fact that free-radical polymerization reactions are rarely driven to completion and small amounts of unreacted monomers or oligomers are present in the cured material.
Due to shrinkage, crosslinked polyester composite materials, such as FRC, generally have poor surface quality or surface “profile”. Typically, large peaks and valleys can be observed on the surface of composite parts. Considerable efforts have been made by resin and part manufacturers to improve the surface profile and dimensional stability of these materials. LPAs, which typically are thermoplastic additives, are mentioned in the art as a means to improve the surface quality of UPR and VER based composite parts. Such thermoplastics include polystyrene, polyesters, polyacrylates, polymethacrylates, polyvinylacetate, polyurethanes, and various polyglycols, and the like. LPA materials substantially improve the surface profile of the composite material by reducing resin shrinkage which results in a material with a smoother surface quality which produces a smoother surface appearance. The surface of the material tends to have a reduced distance between peaks and valleys or a “lower profile”.
Incorporation of LPAs into composite materials may affect mechanical properties of the composite. Thus, the need to incorporate LPAs into the composite material to achieve desired surface quality may limit the use of composite materials for applications where higher mechanical strength is required or conversely in applications requiring high mechanical strength, surface characteristics may be sacrificed due to limitations in the type and/or amount of LPA that can be added to the material.
The art of composite materials constantly seeks materials which provide effective shrink control. We have found that isosorbide-modified unsaturated polyester, and formulations comprising such polyester, incorporated into composite materials, for example FRC materials, provide for improved shrink control and/or surface quality of molded composite parts. While not wishing to be bound to any theory, the inventors believe that the isosorbide-modified unsaturated polyester, and formulations comprising such polyester, may enhance the profiling efficiency of standard LPAs and/or improve surface characteristics of molded composite parts comprising the formulation having isosorbide-modified unsaturated polyester and LPAs.
All parts and percentages set forth in this specification and the appended claims are on a weight-by-weight basis unless specified otherwise.